The essence of an antibody molecule is its Y-shape. By 1940, Pauling envisioned that antibodies have three regions and correctly predicted that the middle part has the same configuration as normal γ-globulin while the two ends have variable configurations that are complementary to the surface of an antigen1) (Non-Patent Document 1). Porter proved in 1958 that γ-globulin is formed from three globular sections, and demonstrated that these sections could be split apart by papain2) (Non-Patent Document 2). The sequence of one part (Fc) of these sections was shown to be essentially conserved in all γ-globulins, while the other two sections (Fab) were shown to vary considerably in sequence from molecule to molecule. By 1969, Edelman et al. presented a complete description of the connections between the Fab region and the Fc region3) (Non-Patent Document 3). Papain cleavage occurs within two heavy chains so that Fab arms, each of which has a light chain bound to the N-terminal portion of the heavy chain by a disulfide, are released from an Fc fragment that is a disulfide-bound dimer composed of the C-terminal half of the heavy chains. All of the cysteines participating in these interchain disulfide bonds are clustered at the very middle of the heavy chain, giving the γ-globulins their Y-shape.
A more dynamic picture of γ-globulin structure has emerged from electron microscopy of antibody-antigen complexes4), 5) (Non-Patent Documents 4 and 5). In the presence of divalent haptens, antibodies form cyclic dimers, trimers, tetramers, pentamers, and larger structures. Although the Fab part and the Fc part have the appearance of rigid rods, the angle between them varies from zero to 180°, allowing them to bridge antigens at distances up to 120 angstroms. The antibody behaves as if all the three parts were bound by a “hinge part” that is a name now used for a heavy chain region containing interchain disulfides. Despite its small size of just ten amino acids in IgG1, the hinge part displays considerable variation in its configuration. The one available crystal structure of a human IgG1 having a full-length hinge part6) (Non-Patent Document 6) reveals extreme asymmetry in the placement of the Fab arms, and this reflects differences in their distance and rotational displacement from Fc. Although the hinge parts on adjacent heavy chains are mutually separated by a distance of 18 angstroms or less, the Fab arms diverge at a 148° angle along their major axes and are rotated by 158° along their depth axes.
At the beginning of 1989, Capon et al. reported that the Fab arms of IgG could be replaced with a variety of other proteins including the extracellular domains of CD4, L-selectin, and tumor necrosis factor (TNF) receptor7) to 14) (Non-Patent Documents 7 to 14). These Y-shaped antibody-like molecules (called immunoadhesins or Fc fusion proteins) are cleaved by papain, like antibodies, into three fragments and have many of the biological properties of IgG including a long plasma half-life, Fc receptor and complement binding, and the ability to cross the placenta. All of them were shown to have therapeutic potential. Specifically, CD4 immunoadhesin prevented HIV-1 infection in chimpanzees, L-selectin immunoadhesin blocked neutrophil influx in mice, and TNF receptor immunoadhesin protected mice against lethal endotoxic shock. Their prolonged half-life in the blood7) (Non-Patent Document 7) has proven particularly valuable, and it leads to the approval of five therapeutic drugs, i.e., etanercept (TNF receptor), abatacept (CTLA-4), alefacept (LFA-3), rilonacept (IL-1 receptor), and romiplostim (thrombopoietin analog)15) (Non-Patent Document 15).